Aromatic hydrocarbon compounds such as benzene are frequently used for producing transportation fuels and petrochemicals such as styrene, phenol, nylon, polyurethanes, and many others. Benzene is typically produced in processes such as steam cracking and catalytic reforming. During steam cracking, a C2+ hydrocarbon feedstock is exposed to high-temperature pyrolysis conditions to produce a product comprising molecular hydrogens, C4− olefin, other C4− hydrocarbons, and C5+ hydrocarbons. The yield of aromatic hydrocarbon from steam cracking is generally much less than the yield of light hydrocarbon. Consequently, complex processes typically are needed for separating and recovering aromatic hydrocarbon from steam cracker effluent. Catalytic naphtha reforming produces a product having a much greater content of aromatic hydrocarbon than steam cracker effluent, but the naphtha feedstock is itself useful for other purposes such as a motor gasoline blendstock.
Various attempts have been made to provide an efficient process for producing aromatic hydrocarbon at high yield from a relatively inexpensive feedstock. For example, processes have been developed for producing light aromatic hydrocarbon (e.g., benzene, toluene, and mixed xylenes—“BTX”) from paraffinic C4− feedstock. The processes typically utilize an acidic molecular sieve, such as ZSM-5, and at least one metal having dehydrogenation functionality, such as one or more of Pt, Ga, Zn, and Mo. These conventional processes typically operate at high temperature and low pressure. Although these conditions are desirable for producing aromatic hydrocarbon, they also lead to undue catalyst deactivation as a result of increased catalyst coking. Also, these conventional processes are not selective for the desirable single-ring aromatic hydrocarbons, such as benzene, toluene and xylene, and produce undesirably large amounts of multiple-ring aromatic hydrocarbons, such as naphthalene.
Therefore, a need exists for processes which convert relatively low-value hydrocarbon feedstock, particularly those processes exhibiting a greater reaction selectivity to the desirable single-ring aromatic hydrocarbon, a lesser reaction selectivity to the undesirable multiple-ring aromatic hydrocarbon, and an acceptable level of feedstock conversion. This invention meets this need and others.